I rented the 100-400 mm for a trip to the San Diego zoo, and thought it was a really nice lens. In terms of utility, though, I'm not sure I have a day-to-day need for it (though need & want aren't always correlated).

As I already own the 70-200 F/2.8 II, I'm wondering if getting the 2X converter is a viable option.

I believe Neuro has previously written that the quality of the 70-200 + 2x is comparable with that of the 100-400. Does anyone have any experience with how fast/accurate the AF when compared?

Moreover, has anyone made this decision before and either been really happy with it or wished they'd made a different choice?

The 100-400 is slightly sharper, and the AF is slightly faster. The 70-200 II + 2x has slightly better bokeh, and while the AF is a little slower, it also hunts less frequently in tough situations (bird in thicket). If I didn't have the 100-400 already, I think I'd be fine with the 70-200 II + 2x. (Although with the new 1D X firmware, I can now AF with the 100-400 + 1.4x, which is quite useful).

Starting arguments on two threads about this topic right now. The 2X mkIII is a much better option. What neuro said about IQ and AF is right but its not a big difference at all. The 70-200 II and 2x III combo has two more stops of IS. When using telephoto lenses IRL anyone would rather have better IS than a hair better IQ and AF unless you spend all day on a tripod or photographing ISO charts. You will spend a lot less time switching lenses as up to 200mm the difference is pretty huge in favor of the 70-200. You will have a lighter bag, better back, less time fumbling with lenses (TC switching is SOOOO much easier than lens switching), $800 and more keepers because of the IS. I call that a pretty clear win.

I recently (12 month ago) had a similar issue.I do own an 70-200 2.8 IS MKII.I do NOT own a 2x-konverterAfter some rental of the 100-400 I decided to go for the 100-400 instead of the 2x-konverter for the following reasons:

- price (in germany, 1 EUro = 1.3 USD approx): 2x-konverter only: 500 Euro / 100-400 optics: 1.400 EURof cause a big difference, but at the 100-400 you get a complete optics which you'll be able to sell for some money as well later.

- handling: i personally do not feel comfortable to frequently change the lenses on location, the converter adds some chances to get dust in the system.

summarizing:the 100-400 is a affordable (somekind) good optics which deliveres excellent results.the 70-200 + 2x is a good workaround which deliveres good results.

Please keep in mind that automatic lens-correctsions (dxo-software, pp) might not be available for the 70-200 +2x-kombination.

My recommendation is the 100-400, if you already do own a 70-200.if not the 70-200 of cause is first.even then in my eyes the 100-400 is the better invenstment.but of cause, it's the money that counts.

I recently (12 month ago) had a similar issue.I do own an 70-200 2.8 IS MKII.I do NOT own a 2x-konverterAfter some rental of the 100-400 I decided to go for the 100-400 instead of the 2x-konverter for the following reasons:

- price (in germany, 1 EUro = 1.3 USD approx): 2x-konverter only: 500 Euro / 100-400 optics: 1.400 EURof cause a big difference, but at the 100-400 you get a complete optics which you'll be able to sell for some money as well later.

- handling: i personally do not feel comfortable to frequently change the lenses on location, the converter adds some chances to get dust in the system.

summarizing:the 100-400 is a affordable (somekind) good optics which deliveres excellent results.the 70-200 + 2x is a good workaround which deliveres good results.

Please keep in mind that automatic lens-correctsions (dxo-software, pp) might not be available for the 70-200 +2x-kombination.

My recommendation is the 100-400, if you already do own a 70-200.if not the 70-200 of cause is first.even then in my eyes the 100-400 is the better invenstment.but of cause, it's the money that counts.

Hmmm... I'm a big (you can read in fat since I'm american although not too fat by american standards), strong and very tall. I photgraph mostly birds because there's not a whole ton else interesting where I live (central connecticut). Therefore I find it easy to handhold big telephotos and run through the woods like a crazy person for the perfect light angle. My tripod stays on my pack a lot and if it can be restrictive for the running around like an a-hole part of my process. I've researched the lens comparisons you referenced many times and the IQ drop is not real important to me (relatively small) compared to the better IS but the AF accuracy is a big deal. I'm getting close to purchasing my medium telephoto lens/lens combo and this question has been on my mind a hell of a lot. The pros seem to be picking the 70-200 with 2x. This will not be my primary lens and I love how the 70-200 looks without a converter and that's huge part of my current thinking and although I am a big guy I'm also psychic and I see a back problem in my future especially because of the 600mm. Given that you've done the research is there anything you can help me out with?

disclaimer: I have not owned the 100-400, but I do have and use the 70-200 II + 2x III combo.

A lot has been written in this thread already, I won't go over any of that. I'll say that if you can shoot at f/8 without issue, the extender combo works great. I've shot this combo on a 7D and a 5D2, work great in either case.

Shawn, the 2xII +70-200will be similar to the 100-400, try renting it. May I add to this post that if you use it for sports and birding, some frustrations will occur in the speed of focusing on a fast moving object vs. The 100-400. Ive used telecoverters for years and they are very usable, but all lenses focus faster without them. So for still photography the converters are very good, but if your main needs are focus speed and 400 mm or bifs, I'd opt for the 100-400is.

I had the 70-200 2.8L IS II and stacked a 2x and 1.4x converter and was getting sharper images than my 100-400L on a 7D. I sold the 100-400. Not long after that, the rest was stolen. Grrrr...

That is strange i own both the 70-200L II, 1.4x Mark III and the 2X Mark III. I recently purchased a 100-400 because the shots with 2X and the 70-200 not sharp enough for me, not to mention how slow the autofocus performs. I dont know if my 2x is defective, but i find the shots of the 100-400 to be much sharper then the 70-200 and the 2X. I think the 70-200 with the 1.4X has better IQ then the 100-400 but as soon as you put that 2X on the 70-200 the 100-400 is better in my opinion (both in IQ and focus speed).

Hello there all on this topic. I use 70-200 II plus 1.4 and 2x. Also 100-400, 400 5.6, 400 f4. Undoubtably 400f4 IS is best route for sublime results. I regularly use 70-200 plus 2x and get excellent images. The only proviso is you must have great light. In the UK only good for limited use however when BIF in Portugal, Spain clear sunny days etc, brilliant. Most versatile and portable combination and sublime lens coupled with 1, 5 or 7D for Candid, portrait, insects, birds, sports, weddings etc. Still get fast AF and bright finder in bright light. Believe me its true. Dont expect to get fast AF on any body lens combo in dull conditions. I have captured fast flight kite surfers, sports and BIF with 70-200 II LIS. I must reiterate, great light=fast AF and sharp images. Failing that USE FLASH. One 580EXII or 600 plus lightweight stand set up strategically will suffice. Oh yes, also constant aperture naked or with 1.4/2x combo.